Democrats, it turns out, read Hal Varian on this question. Here is a summary of the data:
Professors Santa-Clara and Valkanov look at the excess market return – the difference between a broad index of stock prices (similar to the Standard & Poor’s 500-stock index) and the three-month Treasury bill rate – between 1927 and 1998. The excess return measures how attractive stock investments are compared with completely safe investments like short-term T-bills.
Using this measure, they find that during those 72 years the stock market returned about 11 percent more a year under Democratic presidents and 2 percent more under Republicans – a striking difference.
This nine-percentage-point excess can be broken down further into an average 5.3 percent higher real return for the stock market and a 3.7 percent lower return for Treasury bills under Democratic administrations.
This regularity is harder to explain than you think, and simply defending Democrats or attacking Republicans will not do the trick. Remember, high stock market returns mean, not that things are good per se, but rather that things are better than people had expected.
I might have thought that people simply overestimate how bad Democratic Presidents will be. But no, the market does not appear to decline as the election of a Democrat approaches. Nor do changes in the risk premium seem to account for the patterns. Take a look at the original research.
Small companies, by the way, do especially well under Democrats:
One interesting finding is that although both large and small companies do better under Democratic administrations, small companies do especially well, while larger ones do only a little better. The return on the smallest 10 percent of traded companies is 21 percent higher during Democratic administrations, while the return on the largest 10 percent is only 7.7 percent greater.
Email if you have any good ideas. Maybe being unduly pessimistic also causes us to vote for Democrats, that is the best I can come up with. It is in fact the case that conservatives are happier, and more likely to believe that they are in control of their lives, than are liberals. Maybe we see similar patterns, not just in the cross-section, but also across time. When people feel bad, and out of control, stock prices fall too low, and those people act more like Democrats in the voting booth.